Translation:The teachers and principals are following the students.
Sorry, I'm not convinced. As native speaker I agree that this sentence is grammatically correct, but in this example 'lopen te volgen' sounds just very ugly to me. Besides, it sounds like the students are being pursuied rather than their results are being followed.
If you want to emphisize that it's ongoing, you could say "De leraren en directeuren zijn bezig de studenten te volgen" or "zijn de studenten aan het volgen" (I prefer the latter). It's not a rule though, and in your exmple of "Jan loopt te zeuren", "lopen te zeuren" sounds indeed much better than 'Jan is bezig met zeuren". "Jan is aan het zeuren", however, also sounds perfect.
Maybe it is indeed a dialect thing. But then I think Duolingo should not teach it. Perhaps you like to read the following, it's about the use of 'zitten te' instead of 'lopen te'. http://vorige.nrc.nl/rubrieken/woordhoek/article1650477.ece/Zitten_te_lopen_reacties.
Well, whether it is officially regarded as correct or not is besides the point I think. The fact is that it is being used and it is not some local thing. In your link I see references from Drenthe, Twente, Brabant and Vlaanderen. Next to that, your link mentions: "Wel is het duidelijk dat deze werkwoorden een duratief aspect krijgen en eigenlijk op weg zijn naar duratieve hulpwerkwoorden"
So if you want language learners to understand Dutch, you should teach them about this construction and when it is used. AFAIK Duolingo is made to help people learn a language so they can communicate, not to learn them the form of Algemeen Nederlands that is regarded by most as fully correct.
True there are other ways to emphasise something is ongoing. I merely mentioned this, since in your first post you suggest to change it to a sentence without the emphasis that it is ongoing (De leraren en directeuren volgen de studenten), thus with a slightly different meaning.
Onzetaal has an explanation of the construction here.
The lopen te [infinitve] is used to indicate something in an ongoing thing and has been happening for some time:
- ze verkoopt boeken = she sells books/she is selling books (can be used to e.g. indicate a hobby or profession)
- ze loopt boeken te verkopen = she is selling books (emphasizes she is selling now and has been doing this for some time)
The preposition part is part of the verb and when it's an infinitive, it's one word, e.g.:
- wij zijn goed in opruimen = we are good at cleaning (infinitive)
- wij ruimen op = we clean/we are cleaning (conjugated verb)
- de problemen zijn zich aan het opstapelen in de VS = the problems are piling up in the US
- de problemen stapelen zich op = the problems are piling up (zich is in there because it's a reflexive verb)
Removing the preposition in these situations would create a different verb:
- wij zijn goed in ruimen = we are good at removing/killing (used e.g. when all animals in a farm need to be killed due to an outbreak of disease)
- the US sentence wouldn't make sense as stapelen only means literally/phisically putting things in piles, e.g. het kind stapelt de blokken = the child stacks the blocks
@grey236: Maybe I just misunderstand you, but the preposition part is not removed, it is separated in certain cases and then it gets a different position in the sentence, it's explained well here: http://www.dutchgrammar.com/en/?n=Verbs.co04 (you can also have a look at the "next pages" with more explanation on separable verbs).
BTW your sentence should be: Zij loopt op te ruimen.